It is now well known to use a projection light valve system for display of an externally-supplied video signal. There is a problem, however, when a plurality of light valve projectors are utilized to provide overlapping rasters which must be exactly the same horizontal size. In such instances (such as in flight simulators and the like apparatus), easily observable raster size errors can be caused by any error in the relative size of the two overlapped rasters. Accordingly, it is highly desirable to provide a method of, and apparatus for, setting the horizontal size of a light valve projection raster to a desired value and, once set to a desired value, to be maintained.
In some light valve projection systems, the horizontal size of the projection raster may have been established by the frequency of a crystal oscillator. Since crystal oscillators with a high degree of frequency stability can be provided, a similar high degree of horizontal size stability is also obtained.
There is another problem, however, with using crystal oscillator, inasmuch as the color determining, or reference, RF oscillator may be free running, which may result in some frequency drift, with attendent color drift. Accordingly, it is also highly desirable to maintain both the color RF oscillator and the raster size responsive to synchronization provided as part of the externally-supplied video signal to be displayed.